Ground Anchors, soil nails and tension piles, what’s the ...
Transcript of Ground Anchors, soil nails and tension piles, what’s the ...
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Ground Anchors, soil nails and tension piles,
what’s the difference.
Prof. Ir. Jan Maertens , Jan Maertens & Partners
BVBA and KU Leuven
Noel Huybrechts, Belgian building research
institute
1. Introduction.
Actually this is a lot of confusion about the meaning of
the terms ground anchors, soil nails and tension piles or
micropiles working in tension. This was mainly due to the
fact that there is no clear definition and/or
classification of ground anchors. So the difference
between ground anchors, soil nails and tension piles is
not always clear.
The existing European Standards for ground anchors,
micropiles and soil nailing contain a lot of valuable
information. However a lot of cases exist which are not
covered by the existing execution standards and/or by
Eurocode 7.
The fact that GEWI bars and self drilling rods are more
and more used as reinforcement for ground anchors, soil
nails and micropiles working in tension contributes to
the actual confusion.
2. Eurocode 7:
Par. 8.1.1 Scope:
(2) This section is applicable to:
- pre-stressed anchorages consisting of an anchor head, a
tendon free length and a tendon bond length bonded to
the ground by grout;
- non pre-stressed anchorages consisting of an anchor
head, a tendon free length and a restraint such as a
fixed anchor length bonded to the ground by grout, a
deadman anchorage, a screw anchor or a rock bolt.
(3) This section should not be applied to soil nails.
Comment:
In Eurocode 7 the term “anchorages” is used while in EN
1537 the term “ground anchors” is used.
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Par. 8.5.2 Design values of pull-out resistance
determined from the results of tests:
Comment:
There is a clear gap between the anchorages considered in
Eurocode 7 and EN 1537.
It is not clear why Eurocode 7 is also dealing with
anchorages that are not individually checked by
acceptance tests, while EN 1537 is only dealing with
ground anchors that are checked individually by
acceptance tests (= According to EN 1537 passive anchors
can only be considered as ground anchors when every
anchor is tested).
Par. 8.5.3 Design values of pull-out resistance
determined by calculations:
(1) The design value of the pull-out resistance shall be
assessed according to the principles in 2.4.7 and 2.4.8,
where appropriate.
Comment:
The principle is clear, but there is no indication on the
safety factors to be introduced.
3. European Standards Execution of special geotechnical
works:
2.1 EN 1537 – 1999 : ground anchors.
Chapter 1 : Scope
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An anchor consists of an anchor head, a free anchor
length and a fixed anchor length which is bonded to the
ground by grout.
Chapter 3 : Terms, definitions and symbols:
Anchor : an installation capable of transmitting an
applied tensile load to a load bearing stratum.
Figure 1: sketch of a ground anchor – details of anchor
head and head protection omitted.
Chapter
9.7. acceptance test:
Each working anchor shall be subjected to an acceptance
test. The objectives … .
Comment:
According to EN 1537 – 1999 a ground anchor has an anchor
head, a free anchor length and a fixed anchor length and
has to be subjected to an acceptance test.
2.2 EN 14199 – 2003 : Micropiles
Chapter 1: Scope
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1.2. Micropiles are structural members to transfer
actions to the ground and may contain bearing elements to
transfer loads or to limit deformations. ….
Chapter 3: Terms and definitions
3.1. Micropile:
piles which have a small diameter (smaller than 300 mm
outer diameter for bored piles and smaller than 150mm for
displacement piles) and can be installed with small rigs.
Chapter 9: Micropile testing
9.3.2.3.3. unless otherwise specified for micropiles
working in tension at least one static load test should
be performed for the first 25 micropiles and 1 for each
next 25 micropiles.
2.3 prEN 14490 : Soil nailing
Chapter 1: Scope
1.1 Soil nailing is a construction technique used to
enhance / maintain the stability of a soil mass by
installation of reinforcing elements (soil nails).
Typical examples of soil nailing are given in Annex A.
Chapter 3: Terms and definitions
3.15. soil nail:
reinforcement element installed into the ground , usually
at a sub-horizontal angle that mobilises resistance with
the soil along its entire length.
Chapter 9: Supervision, testing and monitoring
9.3.2.1. The frequency and procedures for soil nail load
testing should be based on a consideration of the
consequence of failure, as defined in EN 1990 , EN 1997
and EN 1990.
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9.3.2.2. Table 1 describes the principal types of soil
nail load tests, their purpose, when they are required
and actions to be taken in the event of an non-compliant
test result. Annex A gives guidance on test procedures,
acceptance criteria and the equipment to be used for soil
nail load tests. Table 2 suggests the frequency of soil
nail load tests based on the category of geotechnical
structure.
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4. Actual situation:
When looking to the actual situation a clear difference
should be made between:
- ground anchors with tendon elements of high strength
steel
- ground anchors with tendon elements of low strength
steel
- soil nails
- tension piles
4.1 Ground anchors with tendon elements of high strength
steel:
When tendon elements of high strength steel are used (=
mostly strands) it is clear that prestressing is always
necessary to limit the deformation of the anchors. As in
this case the additional work for the testing of the
anchors is very small testing of all the anchors is
common practice.
For the design of these anchors the safety factors given
in Table A.12 of Eurocode 7 have to be applied. However
it should be clear that the values of Table A.12 can oly
be used if the testing procedure of EN 1537 is strictly
followed. This means that for each jobsite preliminary
suitability tests have to be performed on sacrificial
anchors. This is only common practice for big jobsites.
For small jobsites the pull-out resistance of the anchors
is determined by calculations and is checked by means of
2 or 3 suitability tests on sacrificial anchors who are
installed together with the working anchors or on working
anchors. In this case an R factor comparable to the R
factors given in Tables A6, A7 and A8 of Eurocode 7 for
shaft in tension should be introduced. As the anchors
are individually checked by acceptance tests a
correlation factor must not be applied.
4.2 Ground anchors with tendon elements of low strength
steel:
For the ground anchors with tendon elements of low
strength steel a clear difference has to be made between:
- prestressed ground anchors
- passive anchors that are individually checked by
acceptance tests
- passive anchors that are not individually checked by
acceptance tests
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4.2.1 Prestressed ground anchors:
As for ground anchors with tendon elements of high
strength steel the additional work for the testing of the
anchors is very small. So it may be assumed that testing
of every anchor is common practice.
The same design methods can be applied as for ground
anchors with tendon elements of high strength steel can
be applied.
4.2.2 Passive anchors that are individually checked by
acceptance tests:
Also in this case the additional work for the testing of
the anchors is very small. So it may be assumed that
testing of every anchor is common practice.
The same design methods can be applied as for ground
anchors with tendon elements of high strength steel can
be applied.
4.2.3 Passive anchors that are not individually checked
by acceptance tests:
This type of anchor is not considered in EN 1537.
According to Eurocode 7 the characteristic value should
be related to the suitability test results by applying a
correlation factor a.
It is proposed to apply for this type of anchor the same
safety factors as for tension piles.
4.3 Soil nails:
The execution methods of soil nails and micropiles
working in tension are almost the same. So the difference
beween soil nails and micropiles working in tension is
mostly related to the design method and the assumed load
distribution in the reinforcement element.
When soil nails are used specific methods have to be used
for to calculate the maximum load in the reinforcement
element, the length of the reinforcement element and the
reinforcement of the facing.
Load tests on soil nails can only be performed on
sacrificial nails and on working nails which are debonded
within the zone of influence of the facing. So when soil
nails are used the number of tests to be performed and
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the necessary provisions have to be given in the
specifications.
Due to the completely different design approach for soil
nailing direct comparison of soil nails and ground
anchors or tension piles is not possible.
4.4 Tension piles:
Although the use of tension piles is very common for big
infrastructure works, f.i. MV piles with capacities of up
to 300kN for quay walls, their use is not generally
accepted for the horizontal support of retaining
structures. This is probably due to the fact that there
are no clear rules for the design and testing of tension
piles.
When tension piles are used for the horizontal support of
retaining walls, there will be no load transfer from the
pile to the ground within earth pressure failure zone,
cfr figure 3. Within the earth pressure failure zone the
relative displacement between the pile and the
surrounding soil is always very small. So only the length
of the pile behind the earth pressure failure zone can be
considered.
Figure 3
For the determination of the displacement of the reaining
structure it has to be assumed that no load transfer is
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taking place over the pile length within the earth
pressure failure zone.
The methods normally used to determine the pull-out
resistance of ground anchors can also be used for tension
piles when the same installation methods are applied.
However it should be clear that the same safety factors
cannot be used because tension piles are not individually
checked by acceptance tests. So at least an R factor
comparable to the R factors given in Tables A6, A7 and A8
of Eurocode 7 for shaft in tension should be introduced
and a correlation factor as given in Table A9 of Eurocode
7 to take into account the number of tests performed.
It is also clear that the overall stability has to be
checked in a similar way as for pre-stressed ground
anchors, cfr. figure 4.
Figure 4
When load tests are performed on tension piles installed
for the horizontal support of retaining structures, the
piles have to be debonded over the length of the earth
pressure failure zone. When the length of the piles has
to be increased to obtain enough safety for the overall
stability the debonded length has to be increased to Lt –
Lr, with:
Lt = the total length of the tension pile
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Lr = the minimum length necessary to transfer the load to
the soil
5. Proposal:
5.1 Classification:
In order to cover all types of ground anchors which are
actually installed the following classification is
proposed:
- prestressed ground anchors;
- passive ground anchors;
- tension (micro)piles;
- soil nails.
a) Prestressed ground anchors :
- prestressed ground anchors may have a tendon element
of high strength or low strength steel.
- they always have a free length and a fixed length;
- the fixed length is installed behind the so called
active wedge and in this way that the necessary
factor of safety is available for the overall
stability;
- they are always tested. When testing is not possible
due to an excessive deformation of the retaining wall
or the reaction system a higher value of the safety
factor has to be introduced.
- they are always prestressed by means of a hydraulic
jack.
b) Passive ground anchors:
- passive ground anchors always have a tendon element
of low strength steel;
- they always have a free length and a fixed length;
the fixed length is installed behind the so called
active wedge and in this way that the necessary
factor of safety is available for the overall
stability;
- the number of tests to be performed has to be clearly
specified in the tender documents and/or in the
method statement;
- it has to be demonstrated that the displacement of
the anchor head is smaller than the allowable
displacement of the retaining structure.
c) Tension Piles:
- tension (micro)piles may have a tendon element of
high strength or of low strength steel;
- they have only a fixed length;
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the length of the piles is determined in this way
that the necessary factor of safety is available for
the overall stability,
- the number of tests to be performed has to be clearly
specified in the tender documents and/or method
statement. Tests on working piles can only be
performed for vertical piles. For inclined piles a
free length of min. 2 meters has to be provided ( =
especially installed piles);
- it has to be demonstrated that the displacement of
the pile head is smaller than the allowable
displacement of the retaining structure.
d) Soil nails:
- soil nails have always a tendon element of low
strength steel;
- they have only a fixed length;
- soil nails are used as soil reinforcement and not as
anchors.
5.2 Design:
The ultimate skin friction over the fixed length is
determined taking into account the applied drilling and
injection technique.
The factors of safety to be introduced are determined
based on the type and number of executed tests:
- preliminary tests
- tests on sacrificial anchors
- acceptance tests on working anchors.
5.3 Control tests.
Following types of tests can be considered.
a) Prestressed ground anchors:
- preliminary tests on sacrificial anchors
- suitability tests on sacrificial anchors
- suitability tests on working anchors (= not foreseen
in EC7 but should be allowed for temporary anchors)
- acceptance tests on all working anchors.
b) Passive ground anchors:
- preliminary tests on sacrificial anchors
- suitability tests on sacrificial anchors
- suitability tests on working anchors (= not foreseen
in EC7 but should be allowed for temporary anchors)
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- acceptance tests on working anchors.
c) Tension piles:
- preliminary tests on sacrificial piles
- suitability tests on sacrificial piles or on selected
working piles (provided with a free length for
inclined piles)
d) Soil nails:
- preliminary tests on sacrificial nails
- suitability tests on sacrificial nails.
6. Conclusions:
The information given in the available European Standards
is not precise enough to allow a correct design of all
the types of anchors that are actually installed.
In order to obtain a better agreement with the actual
daily practice it is necessary to extend EN 1537 with
passive ground anchors and to define in Eurocode 7 the
safety factors which have to be taken in to account when
acceptance tests are not performed on all working
anchors. Further on it is proposed to elaborate a
technical document dealing with the use of tension
(micro)piles as anchoring elements.